In high vacuum systems, there are many classes of vacuum pumps and the pumps are often connected in series. For example, a diffusion pump is used in cooperation with a roughing pump which permits the diffusion pump to operate at a very low pressure. In the diffusion pump, supersonic jets of oil vapor drive gas molecules away from a pumped chamber.
A disadvantage of diffusion pumps is that, in the molecular flow regime of very low pressures, there is oil backstreaming from the diffusion pump toward the pumped chamber. In the past, cryogenic cold traps comprising liquid nitrogen cooled baffles have been positioned between the pumped chamber and the diffusion pump. The cryotrap serves as a barrier to backstreaming oil and also serve as an efficient water pumping mechanism for pumping water vapor from the pumped chamber. The cryotrap may be supplemented with water cooled baffles interposed between the cryotrap and the warm diffusion pump to minimize the heat load to the nitrogen cooled trap and to serve as a first barrier to the backstreaming oil.
In recent years, cold traps cooled by closed cycle cryogenic refrigerators such as Gifford-McMahon cycle refrigerators have replaced conventional nitrogen cooled cold traps. Such traps typically serve as water pumps positioned between work chambers and primary pumps such as diffusion pumps and turbomolecular pumps.